The present invention relates to stable thixotropic polysuccinimide (PSI) dispersions, known as slurries, and to their use as additives for improving the flow properties of concrete, in oil wells, in acidic gel cleaners, and in drainage systems of tunnel constructions. Stable in the sense of the present invention means stable against separation of liquid and solid compounds.
Polyaspartic acids and their salts have in recent years attracted increased interest as biodegradable alternatives to polyacrylic acids.
The preparation of polyaspartic acids by a variety of processes has been investigated in depth. For example, the condensation of aspartic acid or ammonium maleate and/or the dehydration products thereof leads to the intermediate polysuccinimide (PSI), which by hydrolysis under alkaline conditions forms salts of polyaspartic acid. If polysuccinimide is exposed to water under initially neutral conditions, then slowly polyaspartic acid is formed, and a pH of 2.5-3 is established. Under acidic conditions at room temperature, the hydrolysis comes to a standstill (Mosig, Gooding, Wheeler, Ind. Eng. Chem. Res. 1997, 36, 2163-2170).
In the course of the investigations on the hydrolysis of PSI as part of the present invention, the influence of particle size and hence also of the water/PSI phase boundary on the hydrolysis rate of PSI itself became clear. The investigations showed that the hydrolysis rate is directly proportional to the particle surface area. Reaction of polysuccinimide and water under increased pressure and elevated temperature produces a mixture of aspartic acid and its oligomers.
Because of their acidic properties, polyaspartic acids are highly effective in the breakup or reformation of deposits, such as those of calcium carbonate. The effect is founded firstly on the pH in the immediate vicinity of the opening succinimides in the PSI, secondly, following neutralization of the polyaspartic acid, on the dispersing action of the polyaspartate anion.
The use of polyaspartic acid in an acidic environment is hindered, however, by its instability under these conditions. Polyaspartic acid is subject to acidic hydrolysis to monomeric aspartic acid, especially under a thermal load (investigations by Bayer A G, brochure “PASP Na-Salz”—polyaspartic acid sodium salt; a new, biodegradable dispersant, Edition 3.99, Order No.: CH 201201). It was possible to show that a 1% strength polyaspartic acid undergoes 61% hydrolysis after 24 h at a pH of 4,77% hydrolysis at a pH of 2, which particularly in the case of oil wells is disadvantageous.
Cleaners very often contain viscosity-increasing substances. Examples of cleaners are given in U.S. Pat. No. 6,083,890 with xanthan as thickener, in U.S. Pat. No. 6,004,916 with ammonium EDTA as complexing agent and a solvent, and also nonionic surfactants as dispersion media, and in U.S. Pat. No. 5,554,320 with phosphoric acid and paraffinsulfonate, fatty alcohol-EP/PO copolymer as surfactant.
With the direct use of polyaspartic acid in oil wells, a problem which is found is that the polyaspartic acid becomes diluted as a result of diffusion following its introduction into the bore hole. As a result, the concentration may fall below optimum ranges.
JP-A 8 169 741 describes the use of powderous polysuccinimide for improving the flow behavior of mortar mixtures with Portland cement. Incorporation in powder form, however, is typical, since uniform distribution within the cement mortar is achieved only by prolonged mixing. Uniform distribution is absolutely necessary, since inhomogeneities lead to problems with the ultimate hardness of the cement. It would be advantageous to provide PSI as a paste or dispersion in water, in which the wetting of the polysuccinimide by water had already taken place and which would therefore prevent the lumping that occurs in the course of stirred incorporation.
The object was therefore to find thixotropic formulations of PSI with water which meet all of these requirements as regards stability and liquefiability.
Stable in the sense of the present invention has been already discussed above but also refers to the feature that the thixotropic dispersions of the invention maintain their useful form or useful chemical/physical nature for a reasonable storage period.
Surprisingly, thixotropic dispersions of PSI with water according to the present invention exhibit all of these properties.